Stem cells are capable of forming at least one, and sometimes many, specialized cell types. Non-embryonic stem cells include, for example, neural stem cells, hematopoietic stem cells, endothelial progenitor cells, and mesenchymal stem cells. Until recently, it was thought that such cells were progenitor cells that could only differentiate into cell lineages derived from the tissue of origin (i.e., that hematopoietic stem cell could only differentiate into hematopoietic lineages). However, several recent studies indicate that these cells can differentiate into cells of different lineages (Ferrari, G. et al, 1998; Gussoni, E. et al, 1999; Rafli, S. et al, 1994; Asahara, T. et al, 1997; Lin, Y. et al, 2000; Orlic, D. et al, 2001; Jackson, K. et al, 2001; Petersen, B. E. et al, 1999; Theise, N. D. et al, 2000; Lagasse, E. et al, 2000; Petersen, B. E. et al, 1999; Theise, N. D. et al, 2000; Lagasse, E. et al, 2000; Krause, D. S. et al, 2001; Mezey, E. et al, 2000; Brazelton, T. R. et al, 2000; Orlic, D. et al, 2001; Bjornson, C. et al, 1999; Shih, C. C. et al, 2001; Jackson, K. et al, 1999; Kawada, H. and Ogawa, M., 2001). Multipotent adult progenitor cells, or MAPCs, for example, home to liver, lung, gut and bone marrow and spleen when transfused into murine recipients, where they differentiate in a tissue specific manner.
Despite recent advances in selection techniques, simple, efficient and highly effective culture conditions for use in the differentiation of stem cells into several terminally differentiated cell types have yet to be developed. Neuronal cells are an example of a cell type for which improved methods of differentiation methods are needed. A number of studies have found that terminal neuronal differentiation requires yet-to-be characterized factors secreted by region-specific glial cells. For instance, Wagner et al found that co-culture of Nurr1 neurons with type-II astrocytes from primary E16 rat ventral mesencephalon, the age and region where endogenous neurons of the substantia nigra have just been born, yielded a significant numbers of functioning dopaminergic neurons (Wagner, J. et al, 1999). Panchision et al created a type-II astrocyte line that supports terminal differentiation of dopaminergic neurons (Panchision, D. M. et al, 1999). Song et al also demonstrated that neural differentiation in vitro occurred when NSCs were co-cultured with brain-derived astrocytes (Song, H. et al, 2002). With the exception of perhaps neuronal stem cells, culture conditions for stem cells have not been shown to consistently produce neuronal cells that have undergone developmentally correct progression throughout the in vitro differentiation process (i.e., show morphological and biochemical changes that temporally correspond to those seen in vivo differentiation).
Obtaining neuronal cells suitable for transplantation will have great use in the treatment of neurodegenerative diseases, such as amyotrophic lateral sclerosis, Alzheimer's, and Parkinson's disease. Stem cells, such as MAPCs, can potentially serve as an unlimited source of neuronal tissue for such purposes. Accordingly, culture conditions that achieve differentiation of stem cells to functional neuronal cells that mimics in vivo differentiation would be desirable.